Ground heat exchangers provide more efficient heating and cooling of building spaces by exchanging heat based on the average yearly temperature of the soil and on a higher thermal conductivity of soil compared to air. The relatively constant temperature of the earth provides a more favorable temperature gradient for heat transfer for both heating and cooling than conventional atmospheric air source systems because the atmospheric air experiences an average daily temperature swing of 20° F. and an average seasonal temperature swing of 80° F. on the North America continent. Heat is rejected to the earth in the cooling mode and absorbed from the earth in the heating mode. Ground heat exchange is synonymous with the terms geothermal, shallow geotheiinal, ground source and geoexchange when used in the context of heat exchange with the earth at the earth's ambient temperature.
Ground heat exchangers can provide direct cooling or heating to a building space when the temperature gradient is positive and the ground loop is large enough to handle the heat load. The ground loop comprises the buried piping for the ground heat exchanger and for distribution of the fluid. For most urban applications, a heat pump is installed on the ground loop to increase the thermal gradient to provide “on demand” efficient heating and cooling to a building space. The heat pump greatly increases the load capacity of the ground loop so that residential and business customers can afford the cost of installing a ground loop on their house or building lot.
Various methods have been developed to exchange heat with the earth. Both vertical and horizontal pipe installations have been used to make subterranean ground loops. Experience has shown that horizontal loops are inefficient ground loops because the shallow depth of burial causes the ambient soil temperature to track the surface ground temperature. Horizontal loops buried below the frost line are, however, excellent for melting snow on pedestrian pathways and removing ice from bridges. Vertical ground loops can be open or closed. An open loop is where at least two wells are complete in a high productivity aquifer and water is circulated from one well to another. This method can be no longer used in urban areas due to drinking water safety standards enacted to prevent aquifer contamination.
The vertical closed ground loop heat exchanger uses piping inserted in a drilled hole in the ground. The configuration of the pipe loop is either side-by-side (U-tube) or concentric. The pipe loop can be made of metal or plastic. Initially, metal pipe loops were used in both concentric and U-tube installations to save capital cost, but experience has shown that metal pipe loop installations eventually fail due to anodic corrosion from conducting telluric or man-made electrical currents from one formation layer to another. Experience has also shown that plastic ground loop installations can last indefinitely, but the local ground temperature will heat up or cool down if the seasonal load is not balanced.
Currently, the most common type of vertical closed loop ground heat exchanger is a U-tube installation, which consists of inserting two lengths of high density polyethylene (HDPE) pipe, with a U-bend joint on the bottom, into a 4 to 6 inch diameter borehole. The borehole depth typically ranges from 150 to 400 feet deep into the earth. To prevent aquifer contamination, the bore hole is backfilled with impermeable grout consisting of a high solids bentonite slurry or neat cement. The grout backfill keeps the piping in thermal contact with the wall of the borehole and provides a permeability barrier to reduce the vertical movement of ground water from one aquifer to another or to prevent surface water contamination of an aquifer.
The vertical, closed-loop, ground heat exchanger typically uses water or a water antifreeze mixture as a working thermal fluid. Refrigerant such as Freon typically is not used due to expense and possible aquifer contamination. The water based fluid is circulated through the closed piping system, which consists of a distribution system to the vertical wellbores. The wellbore loop provides a downward path and an upward path that is arranged in either a U-tube or concentric pipe configuration. The U-tube configuration is about 30-60% as efficient as the concentric pipe configuration because, in the U-tube configuration, the returning fluid will reabsorb about 50% of the heat transfer to the ground on the way back up.
The concentric pipe configuration comprises a smaller diameter pipe arranged concentrically within a larger diameter outer pipe (i.e., the “casing”). The inside surface of the smaller diameter pipe provides a center flow channel and the annulus between the outer surface of the smaller diameter pipe and inner surface of the larger diameter pipe provides an annular flow channel. In most concentric pipe designs, the returning fluid should reabsorb less than 10% of the heat transferred to ground. Reference may be had to U.S. Pat. No. 4,574,875 “Heat Exchanger for Geothermal Heating and Cooling Systems” and US Patent Application 20070029066 “Coaxial-Flow Heat Transfer Exchanging Structure for Installation in the Earth and Introducing Turbulence into the Flow of the Aqueous-Based Heat Transfer Fluid Flowing Along the Outer Flow Channel while Its Cross-Sectional Characteristics Produce Fluid Flows There-along Having Optimal Vortex Characteristics that Optimize Heat Transfer with the Earth”, which describe prior concentric piping designs.
The objective of the concentric pipe design is to maximize the heat exchanged between the bulk fluid in the annular flow channel and the earth. As illustrated in FIG. 3, for heat conduction to the earth, the heat must past through three thermal resistances: (1) the resistance 2 of the fluid boundary layer separating the bulk fluid and pipe wall; (2) the resistance 4 of the pipe material; and (3) the resistance 6 of the grout or slurry backfill. Heat loss can also occur between the center channel and annular channel, which reduces heat exchange with the earth. This undesirable condition is known as thermal short circuit. Minimizing thermal resistance between the bulk fluid and earth and maximizing thermal resistance between the center channel and the annular channel allows more heat to be exchanged for a given temperature gradient between the fluid and the earth. Prior U-tube designs have been particularly inadequate in minimizing thermal short circuit while prior concentric pipe designs have been particularly inadequate in minimizing thermal resistance of the grout and pipe wall.
Vertical, concentric-pipe, ground-loop, heat exchangers are also used as thermal banks for thermal energy storage applications. U-tube designs do not have enough water storage volume or high enough pulse heat transfer to make a thermal bank. Ground loops have greater thermal storage capacity than water tanks and they do not take up any valuable building space. For example, a heat pump can run at night to inject or remove heat from an isolated portion of a ground loop with cheaper electrical rates; then, during the day only a pump circulates fluid from the ground loop thermal bank to handle the heating and cooling loads of the building.
Minimizing the fluid boundary layer thermal resistance 2 requires: (1) maintaining separation between the smaller diameter pipe and the larger diameter pipe to prevent low flow zones in the annular channel and (2) preventing the development of laminar flow in the annular channel. The design in U.S. Pat. No. 4,574,875 disposes spacers (i.e., centralizers) periodically along the outer surface of the smaller diameter pipe to maintain alignment between the smaller diameter pipe and the larger diameter pipe (i.e., to assist in centralizing the smaller pipe within the larger pipe). The spacers have projecting spoke-type contacting fins which are also said to generate an amount of beneficial turbulence in the annular channel.
The design in US Patent Application 20070029066 employs the method of disposing a helically-wrapped turbulence generator along the outer surface of the smaller diameter pipe to generate additional vorticity. Cost effectively manufacturing such a pipe with helical fighting disposed along the entire length of the outer surface has proven difficult and such fighting, and pipe, are easily damaged, making the flighted pipe difficult to insert into a larger diameter pipe. Attaching the fighting as a separate piece to a smooth pipe makes the flighting susceptible to slipping along the outer surface of the pipe, which would allow the smaller diameter pipe to come in contact with the larger diameter pipe, thus creating low flow zones.
Minimizing the thermal resistance 4 of the larger diameter pipe requires using a material that: (1) has minimal wall thickness; (2) has enhanced thermal conductivity; (3) has sufficient mechanical strength to prevent collapse during installation; and (4) does not corrode in soil or degrade in antifreeze environments. Thermoplastic resins such as HDPE and PVC offer sufficient mechanical strength and corrosion resistance but they also have high thermal wall resistances that would classify them as thermal insulators. Metal pipe offers very low thermal resistance, but corrosion resistant alloys are very expensive, and their weight makes them more expensive to ship and more difficult to install. The design in U.S. Pat. No. 4,574,875 prefers the use of plastic for the larger diameter pipe and the design in US Patent Application 20070029066 prefers the use of metal or a fluted plastic for the larger diameter pipe. Neither prior design addresses the mechanical strength of thin pipe walls as a function of bore depth.
Minimizing the backfill thermal resistance 6 requires a slurry composition that: (1) has enhanced thermal conductivity; (2) has low permeability; (3) has sufficiently long set times to allow deployment; (4) is environmentally safe with no organic leachate and less than 1 PPM for all metals as defined by a TCLP (Toxic Chemical Leaching Procedure); and (5) does not substantially dissipate in geologies with high groundwater flow. It is common practice to add silica sand to a bentonite and water slurry to enhance thermal conductivity to approximately 1.4 Btu/hr-ft-F. Reference may be had to US Patent Application 20070125274 “Thermally Conductive Grout for Geothermal Heat Pump Systems”, which describes the use of graphite particles, ranging from 10 to 1000 microns in size, added to slurry in concentrations from 2 to 25% by weight to produce a backfill with thermal conductivity greater than 4 W/m-K (2.3 Btu/hr-ft-F) that has lower permeability. The prior art is inadequate in providing details specifying a backfill composition that would enable sufficiently long set times for deployment and resist dissipation due to high ground water flow rates. The design in U.S. Pat. No. 4,574,875 does not address backfill composition and the design in US Patent Application 20070029066 prefers the use of thermally conductive cement but is not specific in backfill mixture composition, nor does it address permeability, environmental safety or dissipation.
Minimizing thermal short circuit requires that the center channel be sufficiently insulated from the annular channel to prevent significant heat flow between the channels. The design in U.S. Pat. No. 4,574,875 offers no solution while the design in US Patent Application 20070029066 prefers relying on laminar flow in the center channel or using an insulating gas within the smaller diameter pipe, which are both impractical solutions to implement.
The designs in both U.S. Pat. No. 4,574,875 and US Patent Application 20070029066 also fail to provide solutions that: (1) minimize pressure drop across the system; (2) prevent blockage of the center channel outlet; and (3) facilitate installation.